如何使用mgo $ sum运算符

My code:

func (db *MongoConn) Sum(collection string, query Query, field string) (float64, error) {
queryResult := db.Session.
    DB(db.DialInfo.Database).
    C(collection).
    Pipe([]bson.M{{"$sum": field}})}

Error: exception: Unrecognized pipeline stage name: '$sum'ERROR